Catalyst for use in a Fischer-Tropsch process

ABSTRACT

An improved catalyst for the Fisher-Tropsch process, effective at low temperatures and at low catalyst metal loadings on a support, comprises ruthenium and bromine moieties on a support such as gamma alumina.

This is a divisional of application Ser. No. 683,573 filed Apr. 10, 1991now U.S. Pat. No. 5,070,063 which in turn is a continuation ofapplication Ser. No. 361,372 filed Jun. 5, 1989 now abandoned.

This invention concerns an improved catalyst. More especially itconcerns a catalyst for Fisher-Tropsch synthesis, which is a supportedruthenium catalyst.

The Fischer-Tropsch synthesis is a well-established synthetic route fromsynthesis gas (a mixture of hydrogen and carbon monoxide) tohydrocarbons. There are currently two Fischer-Tropsch plants operatingcommercially, both operated by SASOL of South Africa and both usingiron-based catalysts. It is known that ruthenium-based catalysts areactive in the Fischer-Tropsch synthesis (see King et al, Platinum MetalsRev., 1985, 29, (4) 146-154), but for a variety of commercial andtechnical reasons these have not been used on a commercial scale.

The present invention provides a catalyst for Fischer-Tropsch synthesis,comprising a first component of metallic ruthenium and a second andpromoting component of bromine moieties, supported on a third componentwhich is a high surface area support. The support is preferablygamma-alumina, although other alumina or other catalyst supports may beused.

The activity of the catalyst of the invention is such that lower thannormally recommended loadings of ruthenium metal can be used,substantially reducing the capital cost of the catalyst. Furtheradvantages accrue from the use of low metal loadings, and it is believedthat the low levels of methane formation observed in tests withpreferred catalysts can be attributed at least in part to low rutheniummetal loadings. It is also believed that the catalysts of the inventionoffer a better conversion efficiency than a known ruthenium catalyst of3 or 4% metal loadings, and this may be observed at relatively lowtemperature (below 300° C.) Fischer-Tropsch synthesis.

The invention also provides, therefore, the use in a Fischer-Tropschsynthesis of hydrocarbons from hydrogen and carbon monoxide gases, ofthe catalyst of the invention.

The loading of ruthenium metal on the support is suitably 0.05 to 5%,preferably 0.1 to 2%, most preferably 0.1 to 1%, by weight of the totalcatalyst. Suitable atomic ratios of bromine to ruthenium are 0.1 to6.0:1, preferably 1.5 to 4.1.

The catalyst may contain one or more other components; alkali metals oralkaline earth metals, for example potassium, caesium, barium and thelike, in metal or ionic form, may be considered.

Preferably, the gamma alumina has a surface area of 50 to 350 m² g⁻¹,most preferably 150 to 300 m² g⁻¹.

The process of the invention may be carried out at temperatures of from150° to 300° C., preferably in the range 180° to 250° C. Conventionalpressures of synthesis gas may be used, conveniently in the range 40 to120 bar, preferably 50 to 100 bar. Conventional gas flow rates may alsobe used. It is preferred to operate the process using the catalyst in afixed bed reactor.

The catalyst of the invention may be made by impregnating the supportwith sources of ruthenium and bromine moieties, for example by theincipient wetness technique or by spraying. The ruthenium and brominemay be deposited in any order or simultaneously. Thereafter, thecatalyst is treated as necessary to convert at least a major proportionof the ruthenium present to the metallic form. The source of rutheniumand bromine may be, for example, an aqueous solution of a rutheniumbromide, such as ruthenium tribromide. Other single compounds orcomplexes may be used.

The invention will now be described by way of example only, in whichExamples 1 to 10 are of the invention, and Examples A, B and C are givenfor comparative purposes.

EXAMPLE 1

So-called "ruthenium tribromide crystal" was made by dissolvingruthenium hydroxide in aqueous hydrobromic acid and evaporating down toleave a residue crystal which comprises mainly ruthenium and bromidemoieties possibly contaminated with oxy- or hydroxy-moieties. 54 ml ofan aqueous solution of the crystals containing 9.3 g l⁻¹ of rutheniumwere diluted with distilled water to make 116 ml and the dilutedsolution was stirred into 200 g of commercial gamma alumina pellets. Thepellets were cylindrical extrudates of alumina of diameter 1.59 mmchopped into lengths of from 3 to 10 mm. The quantity of solution addedwas sufficient to wet thoroughly the surface of the pellets. The wettedpellets were heated to 140° C. for 3 days in an air oven then 33 g ofcatalyst precursor were loaded into an autoclave basket and theautoclave was heated to 210° C. Wet hydrogen at 1 bar pressure waspassed through the autoclave for 30 minutes at a gas hourly spacevelocity (GHSV) of 500 hr⁻¹. (GHSV is the volume of gas passing throughthe autoclave per hour divided by the volume of the catalyst bed). Thehydrogen reduced the ruthenium compounds to ruthenium metal so producinga catalyst system comprising ruthenium metal and bromine moietiessupported on gamma-alumina. The catalyst contained bromine and rutheniumin an atomic ratio of 2.75:1 and a ruthenium loading of 0.25 wt % of thecatalyst system.

The supported catalyst system was used in the Fischer-Tropsch process asfollows:

The hydrogen supply to the autoclave was replaced by a supply ofFischer-Tropsch synthesis gas comprising

58 vol. % hydrogen

29 vol. % carbon monoxide

and 13 vol. % argon

The argon was used to simulate the presence of the variety of inertgases found in commercial Fischer-Tropsch synthesis gas and to provide aconvenient reference for measuring conversion efficiency by gas phasechromatography. The synthesis gas was supplied at a pressure of 61 barand at an GHSV of 500 hr⁻¹. It was found that conversion of over 40% ofthe carbon monoxide to hydrocarbons could be achieved and only from 0.4to 2.8% of the converted carbon monoxide was converted to methane, inthe temperature interval of 210° to 250° C.

EXAMPLE 2

Example 1 was repeated except that aqueous bromoruthenic acid was usedinstead of ruthenium bromide crystal dissolved in water.

The catalyst contained bromine and ruthenium in an atomic ratio of 4:1and the catalyst contained 0.25 wt % ruthenium. It was found that theconversion of carbon monoxide to total hydrocarbons was again over 40%but this time the conversion of carbon monoxide to methane was evenlower at 0.4 to 1.3% of the total amount of carbon monoxide converted.

COMPARATIVE EXAMPLE A

4.65 g of bromoruthenic acid solution was diluted to 116 ml in water andused to impregnate 200 gamma-Al₂ O₃ pellets (1.5 mm extrudate). Theimpregnated catalyst was dried at 140° C. for 3 days. 50 g of this batchwere then reduced in H₂ and then tested in the Fischer-Tropsch processaccording to the procedures of Example 2. It was found that an optimumconversion of CO to total hydrocarbons of 40% was achieved (catalyst A).

A further 50 g of the batch was then washed by adding to it 250 ml of0.05 molar sodium nitrate solution to remove bromide moieties. Thecatalyst was allowed to stand for 5 minutes in the solution and then thesolution was decanted off and discarded. The washing step was repeatedthree more times. The catalyst was then reduced in H₂ and then tested inthe Fisher-Tropsch process according to the procedures of Example 2. Itwas found that an optimum conversion of CO to total hydrocarbons of onlyabout 22% was achieved (catalyst A1).

The Table shows the pre-test analysis for Ru and Br for these catalysts:

    ______________________________________                                               XRF assays                                                                                            Br/Ru                                          Catalyst Wt % Ru      Wt % Br  atomic ratio                                   ______________________________________                                        A        0.22         0.96     5.5                                            A1       0.23         0.14     0.7                                            ______________________________________                                    

Clearly, the removal of the bromide moities reduced the efficiency ofthe catalyst.

COMPARATIVE EXAMPLE B

This example demonstrates the inferior performance of chloride moietiesas species for use in a supported ruthenium catalyst system for theFischer-Tropsch process.

For the purposes of Comparative Example B, 2.93 g of ruthenium asruthenium trichloride was dissolved in 290 ml of water, and the solutionwas stirred into 500 g of gamma-alumina pellets. Thereafter the productwas dried in an air oven at 105° C. overnight and then reduced using wethydrogen at 210° C. for two hours before being dried at 105° C. Thecatalyst was tested in a Fischer-Tropsch process in accordance with theprocedure of Example 1 at a temperature of 220° C. and a pressure of 22bar. It was found that the catalyst system comprised 0.5 wt % rutheniumyet produced only 8.5% conversion of carbon monoxide to hydrocarbons.

COMPARATIVE EXAMPLE C

For the purposes of Comparative Example C, 5 g of ruthenium nitrosylnitrate (Ru(NO) (NO₃)₃) was diluted with water to 58 ml and the solutionstirred into 100 g of gamma-alumina pellets. The product was dried in anair oven at 105° C. overnight before being heated to 240° C. for 1 hourto decompose the nitrosyl nitrate to form a catalyst precursor. Theprecursor was reduced according to the procedure of Comparative ExampleB and the catalyst system obtained was tested in the Fischer-Tropschprocess also in accordance with the procedure of Example 1 at atemperature of 220° C. and a pressure of 22 bar. It was found that thecatalyst system contained 0.5 wt % of ruthenium yet produced only 6.5 wt% conversion of carbon monoxide to hydrocarbons.

By comparing Comparative Examples B and C with Examples 1 and 2, it canbe seen that substitution of bromide moieties by chloride ornitrogen-containing moieties results in less effective Fischer-Tropschcatalyst systems.

EXAMPLES 3 AND 4

These examples illustrate the effect of a high ratio of bromide moietiesto ruthenium on the effectiveness of the catalyst system.

Varying amounts of bromoruthenic acid in 116 mls of distilled water werestirred into 200 g of gamma-alumina pellets as used in Example 1. Bothbatches of wet pellets were dried at 140° C. for 70 hours in an airoven. The dried pellets were then reduced, analysed for ruthenium andbromine and tested in the Fischer-Tropsch process in accordance with theprocedures of Example 1. The results obtained are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                               Wt % of  Wt % of  Atomic Optimum Conversion                                   Ru in    Br in    Ratio of                                                                             of CO to                                      Example                                                                              Catalyst Catalyst Br:Ru  Hydrocarbons %                                ______________________________________                                        3      0.45     1.6      4.44   *40                                           4      0.47     1.0      2.66   68                                            ______________________________________                                         *Large proportion of methane produced.                                   

It is preferred that the bromide to ruthenium ratio should not exceedabout 4:1 and that the minimum ratio should be about 1.5:1.

EXAMPLES 5 TO 10

These examples illustrate the effects of varying the amount of rutheniumin the catalyst.

Various amounts of bromoruthenic acid in either 92.6 ml or 116 ml ofdistilled water (see Table 2) were stirred into 200 g of gamma-aluminapellets as used in Example 1 except that in the case of Examples 5 to 7the diameter of the cylindrical extrudate was 1.21 mm. The wet pelletswere dried in an air oven at either 190° C. for 18 hours (Examples 5 to7) or 140° C. for 3 days (Examples 8 to 10) to produce a catalystprecursor. The precursor was then reduced to produce a catalyst and thecatalyst was tested in the Fischer-Tropsch process according to theprocedures of Example 1. The results obtained are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________         Concentration                                                                          Wt % Ru in                                                                           Optimum % CO                                                                           % of Converted                                       of HRuBr.sub.4                                                                         catalyst                                                                             converted to                                                                           CO which is                                     Example                                                                            used     system hydrocarbons                                                                           converted to CH.sub.4                           __________________________________________________________________________    5    5.25 g in 92.6 ml                                                                      0.35   72       3.6                                             6    6.0 g in 92.6 ml                                                                       0.40   73       4.0                                             7    7.8 g in 92.6 ml                                                                       0.52   78       6.4                                             8    4.65 g in 116 ml                                                                       0.25   40       1.3                                             9    9.3 g in 116 ml                                                                        0.50   40       2.6                                             10   13.95 g in 116 ml                                                                      0.75   64       4.4                                             __________________________________________________________________________

Higher proportions of ruthenium in the catalyst system and finersubstrate pellets both favour higher conversions of carbon monoxide tohydrocarbons but they also favour higher conversions of carbon monoxideto methane.

I claim
 1. In a Fischer-Tropsch synthesis process for makinghydrocarbons which comprises reacting hydrogen and carbon monoxide inthe presence of a catalyst having a first component of rutheniumsupported on a second component which is a high surface area catalystsupport, the improvement comprising the presence in the catalyst of athird and promoting component of bromine moieties and of the firstcomponent being in the form of metallic ruthenium.
 2. A processaccording to claim 1, carried out at a temperature of 150° to 300° C. 3.A process according to claim 1, carried out at a temperature of 180° to250° C.
 4. A process according to claim 1, wherein the second componentis alumina.
 5. A process according to claim 1, wherein the secondcomponent is gamma-alumina.
 6. A process according to claim 5, whereinthe gamma-alumina has a surface area of from 50 to 350 m² g⁻¹.
 7. Aprocess according to claim 5, wherein the gamma-alumina has a surfacearea of from 150 to 300 m² g⁻¹.
 8. A process according to claim 1,wherein the ruthenium is present in an amount of 0.05 to 5% by weight ofthe total catalyst.
 9. A process according to claim 8, wherein theruthenium is present in an amount of 0.1 to 2% by weight of the totalcatalyst.
 10. A process according to claim 8, wherein the ruthenium ispresent in an amount of 0.1 to 1% by weight of the total catalyst.
 11. Aprocess according to claim 1, wherein the atomic ratio of bromine toruthenium is from 0.1 to 6:1.
 12. A process according to claim 11,wherein the atomic ratio of bromine to ruthenium is from 1.5 to 4:1.